JP5385774B2 - Thermal shock resistant silicon nitride sintered body and method for producing the same - Google Patents

Description

The present invention relates to a silicon nitride sintered body excellent in thermal shock resistance. For example, it is used as a molten metal member that contacts a molten metal.

Since silicon nitride is excellent in heat resistance and hardly wets with metal, it is suitable for a molten metal member into which a molten metal is poured.

A molten metal having a temperature of 500 ° C. or higher is poured into the molten metal member instantly. In some cases, it is necessary to rapidly cool the member itself in order to cool the molten metal. For this reason, the molten metal member is required to have thermal shock resistance. Parameters related to thermal shock resistance include thermal conductivity, thermal expansion coefficient, and strength. Generally, the lower thermal expansion coefficient, lower Young's modulus, higher strength, and higher thermal conductivity, the better the thermal shock resistance. . With regard to silicon nitride, many studies have been conducted with a particular focus on thermal conductivity and strength.

For example, in order to improve the thermal conductivity of silicon nitride, it has been proposed to use an Mg—Y—O-based sintering aid instead of the conventionally used Al—RE (rare earth element) —O system. (See Patent Documents 1 and 2).

In Patent Document 1, since the thermal conductivity of the silicon nitride crystal itself decreases due to the solid solution of Al atoms in the silicon nitride crystal particles and the formation of the sialon phase, the Mg—YO system is used as a sintering aid. An example used is shown. Specifically, silicon nitride is the main component, and the rare earth element and Mg are combined in an oxide equivalent amount of 4 to 30 mol%, and the rare earth metal and Mg equivalent oxide equivalent molar ratio (RE ₂ O ₃ / MgO) is A molded body containing 0.1 to 15 and a relative density of 48 to 56% with an Al oxide equivalent of 1 mol% or less is fired in a non-oxidizing atmosphere at 1500 to 1800 ° C. The silicon nitride material having a thermal conductivity of 50 W / m · K and an intensity of 600 MPa or more with an average major axis diameter of the silicon nitride crystal of 0.5 to 3 μm at the cut surface of the sintered body. It is described that a heat dissipating member is obtained.

Patent Document 2 also shows an example in which an Mg—Y—O-based sintering aid is used, as in Patent Document 1. Specifically, 1 to 50 parts by weight of silicon nitride powder, 99 to 50 parts by weight of α-type silicon nitride powder having an average particle size of 0.2 to 4 μm, a rare earth element including Mg, La, Y, and Yb And a sintering aid comprising at least one kind of rare earth element selected from the above, wherein the Mg is converted into magnesium oxide and at least one kind selected from rare earth elements including La, Y and Yb. Silicon nitride in which the elements of the above are converted into oxides (RE _X O _Y ), the total content of these oxides is 0.6 to 7 wt%, and the weight ratio of (MgO / RE _X O _Y ) is 1 to 70 An elementary sintered body is described.

JP 2000-44351 A JP 2004-262756 A

However, in the inventions described in these documents, although thermal conductivity and bending strength are high, it cannot be said that the thermal shock resistance is sufficient, and a member having higher thermal shock resistance has been demanded. Further, with the Mg—Y—O-based sintering aid, color unevenness may occur in the sintered body, and improvement has been desired.

The present invention has been made in view of these problems, and provides a silicon nitride sintered body having no color unevenness and excellent thermal shock resistance.

The present invention includes a neodymium _iron, the mass ratio represented by _{Fe 2 O 3 / Nd 2 O} 3 is from 0.17 to 10, 0.1 to 0.5 mass ferric converted oxidize iron %, Neodymium 0.05 to 0.59% by mass in terms of oxide, magnesium, yttrium, and neodymium 0.1 to 10% by mass in terms of oxide, MgO / (Y ₂ O ₃ + Nd ₂ O ₃ ) And the area ratio of particles having a major axis diameter of 0.5 times or less of the average major axis diameter by cross-sectional observation of the sintered body is 20% or less, 1 A thermal shock-resistant silicon nitride sintered body characterized in that the area ratio of particles having a major axis diameter of 5 times or more is 25% or more, and the total of these is 30 to 70% . The present invention has been found that the thermal shock resistance is improved by including a predetermined amount of neodymium and iron in a silicon nitride sintered body. By setting it as said range, the liquidus temperature of neodymium oxide falls, the grain growth of silicon nitride is accelerated | stimulated, and densification and a thermal shock resistance improve. Moreover, the color nonuniformity of a silicon nitride sintered compact can be reduced by including neodymium and iron.

It is preferable to contain iron in an amount of 0.1 to 0.5% by mass in terms of ferric oxide and neodymium in an amount of 0.05 to 0.59% in terms of oxide. Thermal shock resistance improves by setting it as the said mass ratio and setting it as such content.

Furthermore, in the present invention, magnesium and yttrium may be included. For example, silicon nitride containing magnesium, yttrium, and neodymium in a total amount of 0.1 to 10% by mass in terms of oxides and having a mass ratio represented by MgO / (Y ₂ O ₃ + Nd ₂ O ₃ ) of 0.5 to 10 It can be a sintered body. By including magnesium or yttrium, the thermal shock resistance is further improved.

In the silicon nitride sintered body of the present invention, the area ratio of particles having a major axis diameter of 0.5 times or less of the average major axis diameter by cross-sectional observation of the sintered body is 20% or less and 1.5 times or more long. The area ratio of particles having an axial diameter is 25% or more, and the total of these is 30 to 70%. By forming such a structure, the thermal shock resistance is dramatically improved. In addition to improving the thermal conductivity, the composite structure as described above maximizes the effect of dissipating fracture energy due to crack deflection (crack deflection) caused by coarse particles. Excellent thermal shock resistance can be obtained.

The degreased body obtained by degreasing the compact of the raw material powder is placed in a container having at least an inner surface made of nitride, and fired at an inert gas atmosphere pressure of 1200 ° C. or higher at 0.5 to 2.0 MPa. It is preferable that it is what was tied. This is to prevent carbon from adhering and suppress the occurrence of color unevenness. Moreover, the inert gas atmosphere is used and the pressure is controlled in order to prevent decomposition of silicon nitride, promote densification, and prevent volatilization of liquid phase forming components such as magnesium oxide. By adjusting the sintering conditions, a silicon nitride sintered body excellent in thermal shock resistance can be obtained. Furthermore, it is more preferable to control the inert gas atmosphere pressure at room temperature or higher.

A silicon nitride sintered body having no color unevenness and excellent thermal shock resistance is provided.

Hereinafter, the silicon nitride sintered body of the present invention will be described in more detail.

The silicon nitride sintered body of the present invention contains neodymium and iron. The present invention has been found that the thermal shock resistance is improved by including a predetermined amount of neodymium and iron in a silicon nitride sintered body. Further, by including neodymium and iron, color unevenness of the silicon nitride sintered body is reduced. Since color unevenness affects radiation, it is preferable that the color tone is uniform in order to equalize the temperature of the member. The reason why the color unevenness is reduced is presumably because the color development is canceled by adding both of them.

The content of neodymium and iron is preferably such that the mass ratio represented by Fe ₂ O ₃ / Nd ₂ O ₃ is 0.17-10. By setting it as said range, the liquidus temperature of neodymium oxide falls, the grain growth of silicon nitride is accelerated | stimulated, and densification and a thermal shock resistance improve. Moreover, if it is such a range, a color nonuniformity can be suppressed.

Moreover, it is preferable to contain 0.1-0.5 mass% of iron in conversion of ferric oxide, and 0.05-0.59 mass% of neodymium in conversion of oxide. By setting the above mass ratio and such a content, the thermal shock resistance is improved, and color unevenness can be suppressed.

Furthermore, in the present invention, magnesium and yttrium may be included. For example, silicon nitride containing magnesium, yttrium, and neodymium in a total amount of 0.1 to 10% by mass in terms of oxides and having a mass ratio represented by MgO / (Y ₂ O ₃ + Nd ₂ O ₃ ) of 0.5 to 10 It can be a sintered body. By including magnesium or yttrium, the thermal shock resistance is further improved.

When magnesium or yttrium is added in the form of an oxide or the like, it reacts with silica present on the surface of the silicon nitride raw material powder to form a liquid phase and promote the growth of silicon nitride particles. By containing in the above range, it can be easily densified.

In the silicon nitride sintered body of the present invention, the area ratio of particles having a major axis diameter of 0.5 times or less of the average major axis diameter by cross-sectional observation of the sintered body is 20% or less and 1.5 times or more long. The area ratio of particles having an axial diameter is 25% or more, and the total of these is 30 to 70%. By forming such a structure, the thermal shock resistance is dramatically improved. This is because, in addition to the improvement of thermal conductivity, cracks are difficult to progress due to the composite structure as described above.

In the silicon nitride sintered body of the present invention, the difference between the average major axis diameter of the sintered body surface and the inside is 10% or less. As described above, the thermal shock resistance can be improved by adjusting the average major axis diameter and making the structure of the surface and the inside of the sintered body uniform. In addition, it is preferable that an average major axis diameter shall be 2-5 micrometers. Within such a range, the silicon nitride sintered body can be densified. Note that the surface of the sintered body refers to a surface that is not burned after sintering. The silicon nitride sintered body of the present invention is excellent in thermal shock resistance and can be used, for example, as a molten metal member in contact with a molten metal. Many of such members are difficult to process such as a flow path of a molten metal, and have a complicated shape, which increases the cost of processing after sintering. By imparting the shape by raw processing, processing after sintering is eliminated, and a member having a surface to be used is used. Therefore, the silicon nitride sintered body of the present invention is suitable for a member having a surface to be burned and requiring thermal shock resistance.

In the present invention, the difference in the content of magnesium contained in the surface of the sintered body and the content of magnesium contained in the sintered body is preferably 1% or less. In order to obtain the crystal structure of the sintered body, it is preferable to reduce the difference in magnesium content between the surface and the inside of the sintered body.

Next, the manufacturing method of the silicon nitride sintered compact of this invention is demonstrated.

The average particle diameter of the raw material silicon nitride powder is preferably 1 μm or less. Moreover, it is preferable to use a silicon nitride raw material powder having a β fraction of 10% or less. Furthermore, since the purity affects the generation of the grain boundary phase and the like, it is preferable that the purity is high, and specifically, it is preferably 98.0% or more. By using such raw material powder, it becomes easy to obtain a silicon nitride sintered body having extremely good thermal shock resistance. In the present invention, the median diameter (D50) obtained by laser diffraction particle size distribution measurement is used as the average particle diameter of the raw material powder.

The raw material powder of silicon nitride preferably contains a certain amount of oxygen. This is to form a liquid phase composed of a complex oxide and an oxide. As oxygen amount, 1-3 mass% is preferable. In this range, silica and iron oxide, neodymium oxide, magnesium oxide, yttrium oxide, etc. present on the surface of the raw material powder form a liquid phase to control grain growth and improve thermal shock resistance. preferable.

For the addition of neodymium, a raw material powder of a neodymium compound such as neodymium oxide, neodymium hydroxide, or neodymium nitrate can be used. As the iron, various powders such as ferrous oxide, iron hydroxide, and nitrate can be applied in addition to ferric oxide. The purity is preferably high purity, and it is desirable to use raw material powder having a purity of 97% or more, more preferably 99% or more. Moreover, it is preferable to use a powder having an average particle diameter of 1 μm or less.

For the addition of magnesium, powders of various magnesium compounds such as magnesium hydroxide and magnesium nitrate can be used in addition to magnesium oxide. Since the purity affects the generation of the grain boundary phase and the like, it is preferable that the purity is high, and it is desirable to use a raw material powder having a purity of 97% or more, more preferably 99% or more. Moreover, it is preferable to use a powder having an average particle diameter of 1 μm or less.

Similarly, when adding yttrium, raw material powders of yttrium compounds such as yttrium oxide, yttrium hydroxide, and yttrium nitrate can be used.

The raw material powder can be mixed by various methods regardless of whether it is dry or wet.

The raw material powder is molded by a molding method such as press molding, CIP molding, or casting. When dry molding such as press molding or CIP molding is used, it is preferable to add a binder to the raw material powder to form granules by a spray drying method or the like.

About the molded object obtained by the said shaping | molding method, degreasing for removing organic substances, such as a binder and a dispersing agent, is performed. Degreasing is preferably performed at 500 to 600 ° C.

Sintering can be produced by a sintering method such as normal pressure sintering, pressurized atmosphere sintering, hot press sintering and the like. The firing temperature is preferably equal to or higher than the temperature at which the transition from α type to β type occurs, and is preferably 1700 to 1900 ° C. When the temperature is higher than 1900 ° C., silicon nitride is decomposed, and when the temperature is lower than 1700 ° C., it may not be sufficiently densified.

The firing atmosphere is preferably an inert gas atmosphere such as argon or nitrogen, more preferably nitrogen. In this case, the pressure is preferably 0.5 to 2.0 MPa. This is for preventing decomposition of silicon nitride and promoting densification. Moreover, it is for preventing volatilization of liquid phase formation components, such as magnesium oxide. In particular, magnesium oxide is easily volatilized, and insufficient densification and color unevenness are likely to occur unless the atmospheric pressure is within a specified pressure range from the beginning of firing. The temperature range controlled in the above atmosphere is preferably 1200 ° C. or higher, where magnesium oxide is likely to volatilize, more preferably 600 ° C. or higher, and most preferably room temperature (25 ° C.) or higher.

Sintering is preferably performed by putting a degreased body in a container having at least an inner surface made of nitride. This is to prevent carbon such as a furnace wall and a heating element from adhering to the degreased body. If carbon adheres, color unevenness tends to occur, which is not preferable. As said container, what consists of nitride sintered compacts, such as silicon nitride and boron nitride, and what apply | coated silicon nitride, BN, etc. to the inner surface of the container of carbon can be used. Furthermore, in the container, 0.01 g / cm ³ or more of the atmosphere forming powder with respect to its volume may be put together with the degreased body and sintered. The atmosphere forming powder may be a powder of the same component as the defatted body, a powder composed of magnesium oxide and other additives that are easily volatilized, or a mixed powder thereof. Use of such an atmosphere-forming powder is preferable for making the structure of the sintered body uniform.

The silicon nitride sintered body of the present invention contains silicon nitride as a main component. The silicon nitride is preferably β-type silicon nitride. α-type silicon nitride is not preferable because of its low thermal conductivity. Therefore, the silicon nitride sintered body of the present invention is composed of β-type silicon nitride particles and a grain boundary phase composed of magnesium oxide or other additives.

Next, the manufacturing method of the silicon nitride sintered compact of this invention is demonstrated.

Hereinafter, the manufacturing method of the silicon nitride sintered compact of this invention is demonstrated using an Example.

[Preparation of sintered silicon nitride]
Silicon nitride raw material powder (average particle size is 1.0 μm, oxygen content is 1.5%, β fraction is 6% or less), neodymium oxide, magnesium oxide source magnesium hydroxide, yttrium oxide, ferric oxide Using the composition shown in Table 1, it was added and mixed. The amount of magnesium hydroxide added was adjusted so that the amount of magnesium oxide contained in the silicon nitride sintered body was a predetermined value. To the obtained mixed powder, an acrylic resin was added as a molding binder, and ion-exchanged water was added as a solvent. After spray drying, molding granules were obtained through a sieve. The average particle size was measured with a laser diffraction particle size distribution measuring machine.

A plate-like molded body of □ 50 × 25 mm was obtained from the obtained granules for molding at a molding pressure of 1.5 t / cm ² .

Degreasing was performed in air at 500 ° C. × 6 hr, 20 ° C./hr.

For firing, a boron nitride sheath is used for the container, and a powder having the same composition as 0.05 g / cm ³ is put into the container, fired in vacuum to a predetermined temperature, and a sintered body of approximately □ 40 × 20 mm is obtained. Obtained. After the predetermined temperature, the atmosphere was adjusted. The sintering temperature was 1750-1900 ° C. Table 1 shows these conditions. Table 1 shows these conditions.

[Evaluation]
The density of the sintered body was calculated by the Archimedes method. The average major axis diameter is measured by mirror-processing an arbitrary cut surface of the sintered body, etching the grain boundary phase in a mixed gas of oxygen and carbon tetrafluoride, and observing with a scanning electron microscope. Calculated using photographs. Specifically, a straight line was drawn at random so as to intersect at least 50 particles, and the major axis diameter was obtained and averaged for all the intersected particles. Moreover, the area ratio of particles having a major axis diameter of 0.5 times or less and 1.5 times or more with respect to the average major axis diameter (represented as A and B in Table 1, respectively) is obtained by calculating the area of intersecting particles. ) Was calculated. About the difference of the average major axis diameter of a sintered compact surface and an inside, the 1 mm cut surface and the 10 mm cut surface were evaluated as the surface and the inside, respectively from the surface of burning. The average major axis diameter of the sintered body is the internal one, and the difference between the average major axis diameter of the surface divided by the average major axis diameter is divided by the average major axis diameter. evaluated.

The thermal shock resistance was evaluated by checking the presence or absence of cracks when the test piece of 4 × 4 × 40 mm was held in air at 1000 ° C. for 1 hour and then dropped into 23 ° C. water. The presence or absence of color unevenness was examined visually. The case where no crack was generated was indicated as “◯”, and the case where crack was generated was indicated as “X”. In Table 1, the production numbers of cracked ones, those having a remarkably low relative density, and those in which a sintered body was not obtained. "*" Was written after the.

Production No. In 2-5, 8-10, 13-17, 20, 23, a dense silicon nitride sintered body having excellent thermal shock resistance and no color unevenness was obtained.

On the other hand, Production No. In No. 1, neodymium was not contained, and thus a silicon nitride sintered body having good thermal shock resistance could not be obtained. Further, color unevenness occurred in the silicon nitride sintered body.

In addition, Production No. In No. 6, since there was too much content of neodymium, the silicon nitride sintered compact with favorable thermal shock resistance was not obtained. Further, color unevenness occurred in the silicon nitride sintered body.

Production No. In No. 7, since the average major axis diameter is small and the area ratio of particles having a major axis diameter of 0.5 times or less of the average major axis diameter exceeds 20%, the sintered body does not become tough. The thermal shock resistance was low. Further, color unevenness occurred in the silicon nitride sintered body.

Production No. No. 11 has a large iron content, a large average major axis diameter, and the area ratio of particles having a major axis diameter of 0.5 times or less of the average major axis diameter exceeded 20%. The body was not toughened and the thermal shock resistance was low. Further, color unevenness occurred in the silicon nitride sintered body.

Production No. In No. 12, the thermal shock resistance was low because a lot of glass phase was produced. Further, color unevenness occurred in the silicon nitride sintered body.

Production No. 18, the area ratio of particles having a major axis diameter of 1.5 times or more with respect to the average major axis diameter is less than 25%, and the major axis diameter is 0.5 times or less with respect to the average major axis diameter. Since the total of the area ratio of the particles having less than 30% was less than 30%, the sintered body was not toughened and the thermal shock resistance was low. Further, color unevenness occurred in the silicon nitride sintered body.

Production No. In No. 19, since the nitrogen atmosphere pressure was small, a sintered body could not be obtained.

Production No. In No. 21, the sintered body was not densified because the nitrogen atmosphere pressure was large.

Production No. No. 22, the average major axis diameter is small, the area ratio of particles having a major axis diameter of 0.5 times or less with respect to the average major axis diameter exceeds 20%, and 1.5% with respect to the average major axis diameter. Since the area ratio of particles having a major axis diameter of twice or more was less than 25%, the sintered body was not toughened and the thermal shock resistance was low. Further, color unevenness occurred in the silicon nitride sintered body.

Production No. 24, the average major axis diameter is large, the area ratio of particles having a major axis diameter of 0.5 times or less with respect to the average major axis diameter, and the major axis diameter of 1.5 times or more with respect to the average major axis diameter Since the total with the area ratio of the particles having the ratio exceeded 70%, the sintered body was not toughened and the thermal shock resistance was low.

In the X-ray diffraction of the silicon nitride sintered body of the present invention, only β-type silicon nitride was detected, and α-type silicon nitride was not detected. In addition, Production No. In 2-5, 8-10, 13-17, 20, 23, the difference of the average long-axis diameter of a sintered compact surface and an inside was 10% or less. Further, when the difference in magnesium content between the surface and the inside was measured by FE-EPMA (Field Emission Electron Beam Microprobe Analyzer, JXA-8500F manufactured by JEOL Ltd.), the production No. 1 was measured. 2-5, 8-10, 13-17, 20, 23 was 1% or less.

Claims (2)

Including neodymium and iron,
The mass ratio represented by Fe ₂ O ₃ / Nd ₂ O ₃ is 0.17 to 10 ,
Containing 0.1 to 0.5% by mass of iron in terms of ferric oxide, 0.05 to 0.59% by mass of neodymium in terms of oxide,
Magnesium, yttrium and neodymium are included in a total amount of 0.1 to 10% by mass in terms of oxides,
The mass ratio represented by MgO / (Y ₂ O ₃ + Nd ₂ O ₃ ) is 0.5 to 10,
The area ratio of particles having a major axis diameter of 0.5 times or less of the average major axis diameter by cross-sectional observation of the sintered body is 20% or less, and the area ratio of particles having a major axis diameter of 1.5 times or more is 25. %, A thermal shock-resistant silicon nitride sintered body having a total of 30 to 70% . A mass ratio represented by Fe ₂ O ₃ / Nd ₂ O ₃ containing 0.1 to 0.5% by mass of iron in terms of ferric oxide and 0.05 to 0.59% by mass of neodymium in terms of oxide. Is 0.17 to 10 and contains 0.1 to 10% by mass of magnesium, yttrium and neodymium in terms of oxides, and the mass ratio represented by MgO / (Y ₂ O ₃ + Nd ₂ O ₃ ) is 0.00. 5 to 10, and the area ratio of particles having a major axis diameter of 0.5 times or less of the average major axis diameter by cross-sectional observation of the sintered body is 20% or less, and the major axis diameter is 1.5 times or more. A method for producing a thermal shock resistant silicon nitride sintered body having an area ratio of particles of 25% or more and a total of 30 to 70%,
In a container having at least an inner surface made of nitride, a degreased body obtained by degreasing the compact of the raw material powder is installed,
A method for producing a thermal shock resistant silicon nitride sintered body characterized by sintering at 1200 ° C. or higher with an inert gas atmosphere pressure of 0.5 to 2.0 MPa.